D1.18 - Not Just for Kids: A Rare Case of Adult-Onset IgE-Mediated Cow’s Milk Allergy Masquerading as Drug Hypersensitivity

Background: IgE-mediated Cow’s Milk Allergy (CMA) is primarily a paediatric condition. While persistence into adulthood is well-documented, de novo onset of CMA in previously tolerant adults is a rare and distinct phenotype. This scarcity of reported cases often leads to diagnostic delays or misattribution to other aetiologies, particularly when reactions occur in the context of co-factors or medication use. We present a case of adult-onset CMA initially mislabelled as beta-lactam allergy, highlighting the complexity of diagnosis and the utility of oral food challenges (OFC) in establishing tolerance to baked products.

Case Description: A 31-year-old female with a history of seasonal allergic rhinitis, presented for evaluation of recurrent anaphylaxis. Symptoms began two years prior, occurring <15 minutes after ingesting raw or lightly cooked dairy. Notably, a severe episode involving generalized urticaria, eyelid angioedema and dyspnoea occurred after co-administering Amoxicillin/Clavulanic acid (Augmentin) with dairy, leading to an initial suspicion of drug allergy. She reported tolerance to dairy throughout childhood and early adulthood.

Diagnostic work-up: Skin prick tests (SPT) for milk and fractions were negative. However, molecular diagnostics revealed specific IgE sensitization to whole milk (cow, sheep, goat) and casein (initially 0.55 kUA/L increasing to 1.35 kUA/L over several months despite avoidance), with negative beta-lactoglobulin and alpha-lactalbumin. SPT and intradermal tests for beta-lactams were negative. To clarify the diagnosis, a drug provocation test with Amoxicillin/Clavulanic acid was performed and resulted negative, ruling out drug hypersensitivity. Subsequently, despite casein sensitization (usually associated with heat stability and intolerance to all forms of milk, irrespective of cooking method), an OFC with extensively heated milk (baked muffin, ~4.4g milk protein) was performed. The patient tolerated the challenge without reaction. Based on these findings and monitoring the trend of specific IgE levels, the next step considered is a challenge with liquid (unheated) milk.

Conclusion: We report a rare phenotype of adult-onset IgE-mediated CMA to casein with preserved tolerance to baked milk. Although our literature review of confirmed cases suggests that adult-onset CMA typically constitutes a distinct phenotype differentiated by isolated sensitization to heat-labile alpha-lactalbumin and beta-lactoglobulin coupled with low IgE to casein, our patient presented a contrasting casein-dominant profile. Remarkably, despite this marker of severity, we demonstrated baked milk tolerance. This case extends the spectrum of adult phenotypes, demonstrating that even casein-sensitized adults can sometimes tolerate baked products. It further underscores the necessity of OFC for accurate management and drug allergy delabelling in complex adult presentations.